1. Field of the Invention
This invention relates to silver halide photographic light-sensitive materials and, more particularly, to spectrally sensitized lith-type silver halide photographic light-sensitive materials (hereinafter referred to as "lith-type photographic materials").
2. Description of the Prior Art
A lith-type photographic material comprises a support having coated thereon a layer of a silver halide photographic emulsion having high contrast and is processed by a specific super high contrast developer (hereinafter referred to as "lithographic developer") to form an image composed of dots and lines having very high contrast, which is used as a photographic original for lithographic printing.
As a technique for producing silver halide photographic emulsions used for such lith-type photographic materials, it is well known to use a spectral sensitization method, that is a technique in which a certain sensitizing dye is added to a silver halide photographic emulsion to render the silver halide sensitive to light sources of long wavelengths.
On the one hand, a sensitizing dye can increase the spectral sensitivity of a lith-type silver halide photographic emulsion, but usually photographic materials containing sensitizing dyes are inferior in dot quality and hence it is very difficult to prepare photographic materials having sufficiently high sensitivity and contrast and provide excellent dot quality. It is already well known, for example, to use polyalkylene oxide compounds to provide excellent dot quality but the dot quality using a polyalkylene oxide compound is frequently liable to deteriorate due to the coexistence of sensitizing dyes.
From another particular nature of lithographic printing, it is advantageous to employ a red light as a safety light during processing. For this reason, it is particularly effective to increase green sensitivity of light sensitive materials. A representative techique for increasing green sensitivity is to use dimethine merocyanine type dyes in light sensitive materials.
Green-sensitized lith-type photographic materials containing dimethine merocyanine dyes, etc., are described in, for example, U.S. Pat. Nos. 4,144,070, 3,703,377 and 4,014,702, Japanese Pat. No. 41204/73, and Research Disclosure, Vol. 161, 16134 (published September 1977). However, the techniques disclosed in the literature are insufficient for obtaining a high green sensitivity and excellent dot quality simultaneously.